1. Field of the Invention
The present invention is concerned with a system for thermoforming masked polymer substrates.
2. Description of the Related Art
Transparent and selectively transparent thermoformable polymeric synthetic resin compositions are employed to form a variety of products such as lenses, windows, housings and a wide-variety of containers and coverings. These polymer compositions are particularly well-suited for automotive use because they provide glass-like optical clarity yet are substantially lighter in weight and offer greater crash safety. High polymer thermoplastic compositions such as polycarbonate, present a superior substrate for molded windshields and windows for vehicles of all types, for example, racing cars, motorcycles, all terrain vehicles, automobiles, heavy equipment, boats and aircraft, as well as for skylights and windows used in conventional housing and mobile homes.
The clear polymer substrate is obtained in sheet form and then subjected to a thermoforming process in which the polymer sheet is placed over a mold and heat is applied. As the heated polymer sheet approaches its melting point, it softens and conforms to the shape of the mold. When cooled, the polymer retains its formed shape. Thermoformable polymers are not abrasion resistant. In order to protect against rough handling during shipping and prior to thermoforming, manufacturers generally apply a protective masking to cover the exposed surfaces. Known protective maskings must be removed prior to the thermoforming process to avoid damaging the polymer substrate or fouling the mold.
One method of masking a polymer substrate involves application of a thin layer of a fibrous material, such as paper, to the surfaces. Such masking is difficult to apply and an adhesive composition must be employed to bond the paper to the polymer sheet. Adhesive bonding agents are incompatible with many polymer compositions and may chemically interact with the polymer, altering the surface of the sheet, for example, with pitting or blistering. Adhesive bonding agents may be also be difficult to remove completely from the substrate surface when the masking is removed. After the adhesive is removed a residue may remain on the surface of the polymer sheet. Adhesively bonded protective masking is not suitable for use during thermoforming and must be removed prior to the application of heat to the polymer substrate in order to avoid melting the adhesive into the substrate.
High polymer films such as low density polyethylene are preferred manufacturer-applied maskings for polymer sheet goods because they can be applied to both planar surfaces immediately following extrusion of the sheet. The elevated temperature of the polymer sheet in combination with applied pressure causes the polyethylene film to bind to the surface of the sheet without the need for an adhesive substance. The film is easily and completely removed by hand peeling without the need for solvents or other mechanical action such as scraping, which might damage the surface of the underlying polymer sheet.
However, because the melting point of such branched chain polyethylenes is substantially lower than that of known thermoformable polymer substrates, polyethylene masking achieves a viscous liquid state at the forming temperatures of polymer substrates. Consequently, the masking must be removed prior to subjecting the substrate to a thermoforming process so that it does not foul the mold. Once the protective masking is removed, the surfaces of the polymer sheet are exposed to damage from abrasion and scratching inflicted by the hard surfaces of the mold during thermoforming. In addition, static electricity may cause the unmasked polymer surfaces to become contaminated with dust and grit during thermoforming and incidental handling.
Where optical clarity of the formed article is important, a hardcoat polymer finish is generally applied to the thermoformed article to protect the surface against scratching. Depending on the intended usage of the article, It may be desirable to apply other coatings such as a tints, photochromic, antifog compositions or the like. Since the surfaces have not been continuously masked during the thermoforming process, acquired particulate matter must be completely removed in an additional cleaning step prior to application of any subsequent coating.
Accordingly, there is a need for a system for thermoforming polymer substrates which are masked to protect the surfaces from damage during the molding process, and which provides a quick, clean release of the finished, masked article.
The present invention provides a thermoforming system for masked polymer substrates which is energy efficient, minimizes thermoforming temperatures and produces a product with superior optical clarity. A temperature resistant, non-stick coating is applied to a sheet of a thermoformable polymer substrate material. The coated sheet is positioned on a flexible platen positioned over a mold having a predetermined shape. Heat is applied until the polymer reaches a thermoforming temperature and softens, whereupon the platen is lowered to stretch over and cover the mold. When the sheet begins to conform to the mold, the heat is withdrawn and a second platen is lowered over the sheet to urge it into conformance with the mold. The temperature resistant coating can be applied over an existing polymer film masking, and rapidly cures to form a non-stick surface which prevents adhesion of the masked polymer sheet to the platen surfaces.
Objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.